* @throws NullPointerException if {@code element} is null
     * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation
     *     would result in more than {@link Integer#MAX_VALUE} occurrences of the element
     */
    @CanIgnoreReturnValue
    public Builder<E> addCopies(E element, int occurrences) {
      requireNonNull(contents); // see the comment on the field